The present invention is related to a method for preventing or reducing the effects of ischemia. The ischemia may be associated with injury, such as occurs as a result of infarctions, thermal injury (burns), surgical trauma, accidental trauma and the like. The ischemia may also precede reperfusion injury. The invention is also related to methods for preventing or reducing bacterial translocation and adult respiratory distress syndrome. In accordance with the present invention, these conditions are prevented by administering dehydroepiandrosterone (DHEA) or DHEA derivatives.
The publications and other materials used herein to illuminate the background of the invention, and in particular cases, to provide additional details respecting the practice, are incorporated by reference, and for convenience-are numerically referenced in the following text and respectively grouped in the appended bibliography.
It has been recognized that the maintenance of vascular integrity is an important response to injury. Complex hemostatic mechanisms of coagulation, platelet function and fibrinolysis exist to minimize adverse consequences of vascular injury and to accelerate vascular repair. Vascular endothelial and smooth muscle cells actively maintain vessel wall thromboresistance by expressing several antithrombotic properties. When perturbed or injured, vascular cells express thrombogenic properties. The hemostatic properties of normal and perturbed vascular cells has been reviewed by Rodgers (1).
Interference with the supply of oxygenated blood to tissues is defined as ischemia. The effects of ischemia are known to be progressive, such that over time cellular vitality continues to deteriorate and tissues become necrotic. Total persistent ischemia, with limited oxygen perfusion of tissues, results in cell death and eventually in coagulation-induced necrosis despite reperfusion with arterial blood. Ischemia is probably the most important cause of coagulative necrosis in human disease. A substantial body of evidence claims that a significant proportion of the injury associated with ischemia is a consequence of the events associated with reperfusion of ischemic tissues, hence the term reperfusion injury. To place reperfusion injury into a clinical perspective, there are three different degrees of cell injury, depending on the duration of ischemia:
(1) With short periods of ischemia, reperfusion (and resupply of oxygen) completely restores the structural and functional integrity of the cell. Whatever degree of injury the cells have incurred can be completely reversed upon reoxygenation. For example, changes in cellular membrane potential, metabolism and ultrastructure are short-lived if the circulation is rapidly restored.
(2) With longer periods of ischemia, reperfusion is not associated with the restoration of cell structure and function, but rather with deterioration and death of cells. The response to reoxygenation in this case is rapid and intense inflammation.
(3) Lethal cell injury may develop during prolonged periods of ischemia, where reperfusion is not a factor.
The reversibility of cell injury as a consequence of ischemia is determined not only by the type and duration of the injury, but also by the cell target. Neurons exhibit very high sensitivity to ischemia, whereas myocardial, pulmonary, hepatic and renal tissues are intermediate in sensitivity. Fibroblasts, epidermis and skeletal muscle have the lowest susceptibility to ischemic injury, requiring several hours without blood supply to develop irreversible damage.
The proximity of the endothelium to circulating leukocytes makes it an important early target for neutrophil adherence and subsequent damage to vascular and parenchymal tissue. Interaction of activated endothelial cells and neutrophils is an immediate early, and necessary, event in ischemia/reperfusion injury (2, 3). The adhesive properties of endothelium are rapidly induced by the influx of oxygenated blood. In response to oxygen, endothelial cells become activated to produce several products, including leukotriene B4 (LTB4), platelet activating factor (PAF) and P-selectin. Leukotriene B4 is a potent neutrophil chemotactic agent (4, 5). Upon activation of the endothelial cells, P-selectin is rapidly translocated from intracellular organelles to the plasma membrane, where it acts to tether circulating neutrophils and stabilize them for activation by endothelial-bound PAF (platelet activating factor), enddothelium-derived cytokines and other biologically active mediators (6). Thus, the physiologic interaction between the activated endothelium and the activated neutrophil is recognized as a critical and immediate early event in reperfusion injury of organs and tissues. Other cellular and biochemical mediators of inflammation injury such as platelets, the complement cascade, and the coagulation system are also important, but come into play much later in the cascade, in a process called coagulative necrosis. Finally, monocytes, macrophages, fibroblasts and smooth muscle cell infiltration are responsible for reconstruction and replacement of dead tissue with new, vital tissue, a process called wound healing.
A popular theory postulates a role for partially reduced, and thus activated, oxygen species in the initiation of membrane damage in reperfusion injury. Present evidence indicates that activated oxygen (superoxide, peroxide, hydroxyl radicals) is formed during ischemic episodes and that reactive oxygen species injure ischemic cells. Toxic oxygen species are generated not during the period of ischemia itself, but rather on restoration of blood flow, or reperfusion. Two sources of activated oxygen species have been implicated as early events in reperfusion injury, those produced intracellularly by the xanthine oxidase pathway and those which can be transported to the extracellular environment by activated neutrophils (2, 3, 7-9).
In the xanthine oxidase-dependent pathway, purines derived from the catabolism of ATP during the ischemic period provide substrates for the activity of xanthine oxidase, which requires oxygen in catalyzing the formation of uric acid. Activated oxygen species are byproducts of this reaction. The species of oxygen radicals derived from the xanthine oxidase pathway are O.sub.2 -- (superoxide with one electron) and H.sub.2 O.sub.2 (hydrogen peroxide with two unpaired electrons). Superoxides are generated within the cytosol by xanthine oxidase (located in the cytosol). The superoxides are then catabolized to peroxides within mitochondria by superoxide dismutase. The peroxides are further converted to water either by glutathione peroxidase, in the cytosol, or by catalase in peroxisomes. Both glutathione peroxidase and catalase comprise the antioxidant defense mechanism of most cells. The major evidence for this hypothesis rests on the ability of allopurinol, an inhibitor of xanthine oxidase, to protect against reperfusion injury in experimental models.
In the NADPH-dependent pathway, NADPH oxidase is activated to generate superoxides through reduction of molecular oxygen at the plasma membrane. The superoxides are reduced to hydrogen peroxide by superoxide dismutase at the plasma membrane or within phagolysosomes. Finally, hydrogen peroxide within phagolysosomes can be reduced in the presence of superoxides or ferrous iron to hydroxyl radicals. A third form of oxygen metabolite is mediated by myloperoxidase in the presence of chlorine to reduce hydrogen peroxide to hypochlorous acid.
The hydroxyl radical is an extremely reactive species. Mitochondrial membranes offer a number of suitable substrates for attack by OH.sup.- radicals. The end result is irreversible damage to mitochondria, perpetuated by a massive influx of Ca.sup.2+ ions. Another probable cause of cell death by hydroxyl radicals is through peroxidation of phospholipids in the plasma membrane. Unsaturated fatty acids are highly susceptible targets of hydroxyl radicals. By removing a hydrogen atom from fatty acids of cell membrane phospholipids, a free lipid radical is formed. These lipid radicals function like hydroxyl radicals to form other lipid peroxide radicals. The destruction of unsaturated fatty acids of phospholipids leads to a loss in membrane fluidity and cell death. Some investigators believe that the effects of oxidative stress cause programmed cell death in a variety of cell types.
Infarctions and traumatic injury involve many tissues, including vascular tissue. One response following traumatic injury is to shut down blood supply to the injured tissue. A purpose of this response is to protect the patient from the entry of infectious agents into the body. The severe reduction in blood supply is a main factor leading to progressive ischemia at the region of the traumatic injury. With progressive ischemia, tissue necrosis extends beyond the directly affected tissue to include surrounding unaffected tissue. This progressive ischemia plays an important role in defining the ultimate tissue pathology observed in humans as a consequence of the traumatic injury. For example, see Robson et al. (10).
One form of traumatic injury which has received a great deal of attention is thermal injury or burns. The burn wound represents a non-uniform injury, and the spectrum of injury ranges from tissue which is totally coagulated at the time of injury to tissue which is only minimally injured. Between these two extremes is tissue which is seriously damaged and not immediately destroyed, but which is destined to die. The etiology of the progressive depth of necrosis has been shown to be stasis and thrombosis of blood flow in the dermal vessels, causing ischemia and destruction of epithelial elements. This ischemia occurs for 24-48 hours following the thermal injury (10, 11). Many effects have been seen following a thermal injury, including adhesion of leukocytes to vessel walls, agglutination of red blood cells and liberation of vasoactive and necrotizing substances (11).
It has been established that burn-associated microvascular occlusion and ischemia are caused by the time dependent increase in development of microthrombi in the zone of stasis, a condition which eventually leads to a total occlusion of the arterioles and a microcirculatory standstill. Whereas margination of erythrocytes, granulocytes and platelets on venular walls are all apparent within the first few hours following thermal injury, the formation of platelet microthrombi (occurring approximately 24 hours after surgery) is believed to be responsible for creating the conditions that cause complete and permanent vascular occlusion and tissue destruction (12, 13). The formation of platelet microthrombi appears to provide the cellular basis for expanding the zone of complete occlusion and the ischemic necrosis that advances into the zone of stasis following thermal injury.
Much effort has been made toward improving the care of burns and other traumatic injuries, and many approaches have been proposed toward reducing the progressive ischemia associated with such injuries. The anti-inflammatory agents indomethacin, acetylsalicylic acid and methylprednisone acetate have been shown to preserve dermal perfusion (10). Three thromboxane inhibitors, imidazole, methimazole and dipyridamole, have been shown to prevent vascular changes in the burn wound, allow dermal perfusion and allow other prostaglandin synthesis, which would circumvent detrimental effects of the anti-inflammatory agents (11). Therapeutic doses of ibuprofen and imidazole were found to prevent dermal vascular occlusion by acting as an antagonist to a plasmin inhibitor (14). The reduction of circulating fibrinogen, shown by administration of ancrod (a pit viper venom), led to preservation of vascular potency at the site of the injury (15). It has also been found that the inhibition of leukocyte-endothelial adherence, shown by using monoclonal antibodies, prevents burn extension/progression in the marginal zone of stasis (16).
Bacterial translocation is the process by which indigenous gut flora penetrate the intestinal barrier and invade sterile tissue. Included in this process is the migration of microbial organisms to the draining mesenteric lymph nodes, spleen, liver, blood and in some instances, the lung (17, 18). This phenomenon has been documented in humans following thermal injury (19-21) and ischemia-reperfusion injury (22).
Under normal conditions, the intestinal mucosa is impermeable to potentially harmful materials from the intestinal lumen (17, 22, 23). Current data supports the concept that disruption of the integrity/permeability of the mucosa promotes bacterial translocation, since exposure to stress which produces a host response characterized by cellular damage and necrotic tissue correlates with development of bacterial translocation (23). The clinically important repercussions of bacterial translocation are sepsis and multi-system organ failure (22-24). The incidence of sepsis and disseminated organ involvement following stress is greatest among patients that also exhibit compromised immune defenses (22, 23), such as observed in thermally injured individuals (24, 25). Thus, in response to stress, some patients demonstrate bacterial translocation in the absence of severe consequences. The patients in this category are those who have retained intact immune defenses (22-24). Because of the well known modulation of the host immune defenses following severe burn, bacterial translocation is one of the more serious consequences of thermal injury in humans (24, 25).
Experimental models of bacterial translocation have noted that irreversible cellular injury of the gut may require up to 24 hours post-thermal injury and 48 hours to visualize histological changes in gut vascular tissue (21, 26). These experimental systems have been useful in defining the pharmacologic mediators which appear to formulate a cascade of effector molecules responsible for tissue necrosis. In addition to the role played by catecholamines, oxygen-free radicals and endotoxin, factors such as interferon alpha, interleukin-6, tumor necrosis factor, platelet activating factor, and many of the vasoactive fatty acids derived from arachidonic acid metabolism have been implicated (17). The contribution of oxygen-free radicals, endotoxin, prostaglandins and thromboxanes in promoting tissue destruction has been supported by the evidence that inhibition of bacterial translocation and mucosal injury has been achieved using allopurinol (27) (an inhibitor of xanthine oxidase), endotoxin desensitization (28), prostaglandin analogs (29) and thromboxane synthetase inhibitors (30).
The evidence implicating the role of neutrophils in adult respiratory distress syndrome (ARDS) is substantial but indirect (31). Some of the first suggestions that neutrophils may cause an ARDS-like picture were found in severely neutropenic patients who were infused intravenously with donor neutrophils. Occasionally, within hours of neutrophil infusion, there was an abrupt "white-out" of the lungs (by x-ray) and onset of ARDS symptoms. Numerous studies have shown that neutrophils accumulate in the lung during ARDS. For example, their presence has been demonstrated histologically. During the early phases of ARDS, the number of circulating whole blood cells transiently decreases, probably due to their abnormal pulmonary sequestration. Some neutrophils that accumulate within lung capillaries leave the vascular space and migrate into the interstitium and alveolar airspaces. In normal healthy volunteers, neutrophils account for less than 3% of the cells that can be obtained by bronchoalveolar lavage (BAL). In patients with ARDS, the percentage of neutrophils in the lavage is markedly increased to 76-85%. The accumulation of neutrophils is associated with evidence of their activation. They demonstrate enhanced chemo- taxis and generate abnormally high levels of oxygen metabolites following in vitro stimulation. Elevated concentrations of neutrophil secretory products, such as lactoferrin, have been detected in the plasma of patients with ARDS. Further evidence that neutrophils actively participate in lung injury was obtained from a clinical study of patients with mild lung injury who were neutropenic for an unrelated reason (e.g., receiving chemotherapy). It was noted that lung impairment frequently worsened if a patient's hematological condition improved and circulating neutrophil counts recovered to normal levels.
Although the evidence implicating neutrophils in the genesis of human ARDS is still largely indirect, data demonstrating the importance of neutrophils in various animal models of acute lung injury is convincing. The common approach that has been used to demonstrate neutrophil independence is to deplete the animal of circulating neutrophils and measure any diminution in lung injury that occurs. Although a number of experimental models have been used to study neutrophil dependence of lung injury, only a few have been selected for discussion herein because of space limitations.
One extensively studied model is the administration of endotoxin to sheep. When endotoxin is intravenously infused into sheep, a complex set of events occurs, one of which is increased permeability of the pulmonary capillary endothelium. This is manifested by an increase in the flow of lung lymph which contains a higher-than-normal protein concentration. These changes indicate a reduction in the ability of the capillary endothelium to retain plasma proteins within the vascular space. The neutrophil dependence of the permeability injury was established when it was found that neutrophil depletion of the sheep prior to endotoxin infusion protected them. Another in vivo model of acute lung injury involves the intravenous infusion of cobra venom factor into rats, which causes complement activation followed by leukoaggregation and sequestration of neutrophils within the pulmonary microvasculature. Alveolar wall damage occurs, leading to interstitial and intra-alveolar edema with hemorrhage and fibrin deposition. Again, neutrophil depletion prevented the increased pulmonary capillary leak.
Isolated, perfused rabbit or rat lungs have also been used to study mechanisms of alveolar injury under circumstances that allow improved control of the variables that affect fluid flux. When neutrophils were added to the perfusate and then stimulated, albumin leaked from the vascular compartment into the lung interstitium and alveolar airspaces. Unstimulated neutrophils or stimulus alone (e.g., phorbol myristate acetate) failed to increase alveolar-capillary permeability.
As further proof that stimulated neutrophils can independently injure lung tissue, in vitro experiments have been performed using vascular endothelial and lung epithelial cells as targets. In some reports, neutrophils have been shown to detach endothelial cells or alveolar epithelial cells from the surface of the tissue culture dish. Obviously, if such an event were to occur in vivo, the denuded surfaces would permit substantial leakage of plasma contents. Furthermore, many reports have provided clear evidence that stimulated neutrophils are able to facilitate lysis of cultured vascular endothelial cells and alveolar epithelial cells.
DHEA is an endogenous androgenic steroid which has been shown to have a myriad of biological activities. Araneo et al (32) has shown that the administration of DHEA to burned mice within one hour after injury resulted in the preservation of normal immunologic competence, including the normal capacity to produce T-cell-derived lymphokines, the generation of cellular immune responses and the ability to resist an induced infection. Eich et al (33, 34) describes the use of DHEA to reduce the rate of platelet aggregation and the use of DHEA or DHEA-sulfate (DHEA-S) to reduce the production of thromboxane, respectively.
Nestler et al. (35) shows that administration of DHEA was able in human patients to reduce body fat mass, increase muscle mass, lower LDL cholesterol levels without affecting HDL cholesterol levels, lower serum apolipoprotein B levels, and not affect tissue sensitivity to insulin. Kent (36) reported DHEA to be a "miracle drug" which may prevent obesity, aging, diabetes millitus and heart disease. DHEA was widely prescribed as a drug treatment for many years. However, the Food and Drug Administration recently restricted its use. DHEA is readily interconvertible with its sulfate ester DHEA-S through the action of intracellular sulfatases and sulfotransferases.
Despite the above discoveries concerning effects of various compounds on burns, there is a need to identify additional compounds which are able to prevent or reduce reperfusion injury as a consequence of ischemia, effects of ischemia associated with infarctions or traumatic injury, and to identify compounds which are able to prevent or reduce bacterial translocation and ARDS. Thus, it is an object of the present invention to prevent or reduce progressive tissue necrosis, to prevent or reduce reperfusion injury, to prevent or reduce bacterial translocation, and to prevent or reduce ARDS.